1. Field of the Invention
The present invention generally relates to image processing apparatuses and image processing methods, and more particularly to an image processing apparatus and an image processing method for processing image data including a memory color, a program causing a computer to execute such a method, and a computer-readable recording medium on which such a program is recorded.
2. Description of the Related Art
Progress in digital technology has made it common to output image data input through input apparatuses such as a digital camera and a network scanner from output apparatuses such as an ink-jet printer, a laser printer, and a digital proofer. Generally, image data input from an image data input apparatus is subjected to a variety of processing by an image processing apparatus or an image processing method causing the image processing apparatus to operate, and is output.
Under such circumstances, a variety of images are processed so that a wide variety of requests as to image quality have been made to image data. Particularly, the reproduction of colors referred to as memory colors such as skin color (skin tones), grass green, and sky blue, which are frequently seen in our daily life, is considered to be important.
Particularly, the skin color of a human image is an image quality factor that attracts the most attention. Accordingly, techniques for correcting the skin color of a human image to an appropriate color have been disclosed.
For instance, according to Japanese Laid-Open Patent Application No. 2000-242775 (Prior Art 1), a facial area is extracted from an image including a man, and color density information representing color and/or density (color density) around the facial area is detected. The color density of the facial area is adjusted based on the color density information. The adjusted image is reproduced so that the facial area of an appropriate color density unaffected by the color density around the facial area is perceived.
Meanwhile, according to Japanese Laid-Open Patent Application No. 2001-186323 (Prior Art 2), a skin-colored area is extracted from a human image included in image data, and the skin color of the extracted skin-colored area is compared with a prepared skin color correction target value to calculate a skin color correction. The color of the skin-colored area of the human image is corrected based on the calculated skin color correction. Thereby, the skin color of a man in an identification photograph can be made closer to an actual skin color.
Further, it is well know that in recognizing the color and the density of a certain area, human vision is affected by color and density around the area, as described, for instance, in “Color Appearance Model,” M. D. Fairchild, ADDISON-WESLEY, 1998. That is, even if the density of an area remains the same, the area seems bright if it is dark around the area, and the area seems dark if it is bright around the area.
For instance, when areas of the uniform gray density are observed against a background having the same gray density as those areas, those areas are perceived as having the same density. On the other hand, when the areas are observed against black and white, respectively, the areas are perceived as having different densities depending on their respective background colors.
Accordingly, even when only a skin-colored area is processed to have an appropriate color and density as described in Prior Art 2, a perceived facial area may be affected by the color and the density of the peripheral area to have an inappropriate color and density.
Further, according to Prior Art 1, the color and the density of a skin-colored area is appropriately corrected considering the color and the density of its peripheral area. However, a preferable color is prevented from being reproduced in some cases.
For instance, in the case of an image in which a skin-colored area exists in a preferable range as a whole and its peripheral area is tinged with blue as a whole, color correction to reduce blueness, that is, add yellowness, is performed on the entire image so as to perform correction to balance chromaticness over the entire image (so-called white balance correction).
The same color correction is performed on the skin-colored area so that the color of the skin-colored area may be offset from the center of a preferable skin color. A description is given, with reference to FIG. 1, of this problem.
FIG. 1 is a diagram for illustrating color correction according to Prior Art 1. In FIG. 1, a skin-colored area and an area of a color other than a skin color (hereinafter referred to as a background area) that are extracted from image data are plotted in the CIELAB uniform color space.
Referring to FIG. 1, the psychometric chroma coordinates of chromaticness are shown two-dimensionally. The horizontal axis represents chromaticness in the red-green direction (a*), and the vertical axis represents chromaticness in the yellow-blue direction (b*). Point S represents the skin-colored area of the image data before correction, and Point R1 represents the chromaticness of the background area before correction. It is assumed that the chromaticness of the skin-colored area of the image data exists in a preferable skin-color range (indicated by hatching). Naturally, in this case, the skin-colored area (point S) is perceived as a preferable skin color.
However, in the case of image data tinged with blue over the entire image whose background area before correction is indicated by Point R2, correction to reduce blueness is performed on the entire image. Accordingly, correction to add yellowness to the entire image data is performed on the entire image. As a result, referring to FIG. 1, the background area is corrected from Point R2 to Point R2′.
Meanwhile, the same color correction is performed on the image data of the skin-colored area so that the skin-colored area is corrected from Point S to Point S′. That is, although the colors are balanced over the entire image, the color of the skin-colored area is slightly more yellowish than a preferable skin color in the obtained image. That is, the skin-colored area of this image data is not perceived as a preferable skin color.